1. Field of the Invention
The present invention relates to power switching relays and more particularly to a power switching relay including means for indicating the state of the relay contacts with respect to the state of the magnetic coil.
2. Description of the Background Art
A background art search directed to the subject matter of this application in the U.S. Patent and Trademark Office disclosed the following U.S. Pat. Nos. 4,105,882; 4,479,117; 4,567,477; 4,645,886; 4,647,727; 4,854,286.
The first patent, 4,105,882, discloses a pressure actuated reversing switch in which manually exerted pressure causes the electrical contacts to make and break in a staggered sequence. The included complex motion permits power contacts to be broken prior to breaking of the contacts at the other end of an included armature. This arrangement permits a self-cleaning action which assures reliability and long life.
U.S. Pat. No. 4,479,117 discloses an add-on fault indicator for a relay comprising a magnetic reed switch and a lamp connected in series across relay contacts with a reed switch mounted contiguous to the relay coil. The reed switch is operated by the magnetic field when the coil is energized. A normal operation indicator lamp is connected in series with the relay contacts to indicate closure of the latter to energize a load. A fault indicator lamp lights if the contacts do not close when the coil is energized but is shunted by the contacts to remain unlighted when the contacts close properly.
U.S. Pat. No. 4,567,477 discloses a smoke detector indicator with an ON-OFF switch indicator system that permits the audio alarm to be turned off for false alarm situations. It further provides a visual indication that the system is not in operation. The included circuit prevents the alarm from sounding, and a visual indicator associated with the switch provides a visual indication when the circuit is open, and is not visually apparent when the circuit is closed.
U.S. Pat. No. 4,645,886 discloses a switch for providing a safety function when its main contacts are fused or stuck together in the ON position. Each arm has a first end which carries the movable main contact and a second end with spring means which mechanically are coupled to the movable switch arm to bias the movable main contact toward the open position away from engagement with the ON contact. Auxiliary safety contacts positioned proximate to the second end include a safety circuit for deenergizing the main circuit in response to abnormal movement of the second end of the movable switch arm.
U.S. Pat. No. 4,647,727 also discloses a safety switch for automatically interrupting a circuit when its main contacts are fused or stuck together in the ON position. The auxiliary safety contacts become opened in response to abnormal movement of the switch arm under influence of a spring when the actuator is moved to the OFF position.
U.S. Pat. No. 4,864,286 discloses monitoring apparatus for a switch, detecting abnormal operation of the actuating mechanism when actuating a switch such as a power circuit breaker. A normal stroke is taken into calculation against a reference stroke and compared with actually measured characteristic stroke to decide whether the operation of the mechanism is correct.
Relays typically can be considered as open-loop subsystems, meaning that once energy is applied to, or removed from, its electromagnetic coil, the state of the relay contacts cannot be directly determined. Because of this, several problems arise from this open-loop nature which leads to the overall system's inability to detect, respond to, or compensate against relay contact failures.
The first one of these is that instance when the coil winding has been stressed or fractured and becomes electrically open thus preventing the moving contacts from closing. Obviously, this creates performance difficulties for a single-relay system as well as for multi-relay systems in which relays operate in concert or combination.
Similar to the first instance is the failure of the coil drive circuitry. Here, the contacts may remain in their "open" state, or, conversely in the activated state, depending upon the effect of the drive circuitry failure.
Finally, when contact welding occurs, welding being that condition when the electromagnetic coil relinquishes its ability to open the contacts, the contacts remain closed. Not only do the problems mentioned above relating to the open coil or coil drive circuitry failure occur, but safety issues now arise.
An operator may attempt to deactivate a relay and without knowing that the relay contacts have welded or stayed in the closed position may expose himself to a hazardous situation.
If a direct method of detecting relay failure existed, a warning light or annuciator could be used as a means to alert the operator of a problem. In addition, electronics could be developed which intelligently respond to failure modes by, for example, removing power from the system to prevent additional relays from being turned on.
At the present time, two methods are typically utilized to compensate for the open loop nature of relays. The first method is to indirectly sense the contacts state by monitoring the existence or non-existence of the anticipated response to the relay such as a motor turning on or a valve closing. It has been found that usually the system to accomplish this type of feedback is quite expensive. A second method is demonstrated performance reliability of the system to a desired level of confidence. Safety agencies such as the Underwriters Laboratory utilize this method to ensure that relays are capable of working with the particular load device throughout the entire life of the product. Nevertheless, under these conditions failures can still occur.
Accordingly, it is the object of the present invention to provide a relay design incorporating circuitry for an independent, integral indication of the state of its moving contacts and thus provide the necessary feedback required for a closed loop relay system.